|
If you buy one of the new units be sure it has the WAAS feature. Wide Area Augmentation System(?)(might be service) makes the unit accurate to about 3 meters (about 10 ft) instead of about 10 meters. I have an old Eagle Explorer and I have been using it to try to find corners on my Father's farm which was surveyed in '33. You will find that you can do better if you change your display to read UTM. It's actually a lot more accurate, but it means learning a little about it. You get two rows of numbers, the last few digits are the distance from a North-South line and an East-West line. I record the last 5 digits of each for each corner, then go to the next corner and do the same. I use my HP calculator to convert the coordinates of the two points into a vector, a line of some length with an angle. The angles will be in true, not magnetic degrees. Some more considerations: If you are trying to find points on a map you will have to change the GPS datum to the datum of the map, or it won't work out right. If you go back the next day and take readings at the same points, your readings will be a lot different. By taking many readings and calculating an average you start to approach the true reading. (Try averaging degrees, minutes, and seconds and you see why UTM is easier) If you are trying to mix in readings from a compass you need to throw in the magnetic deviation, or you could be off several degrees, like about 7, I think for my area. So if you are looking at a plat you need to find the mag. dev. for the year and month it was done. Without an auxiliary antenna it won't work very well under heavy tree canopy. Try googling for gps antennas. I have seen plans for several homebrew antennas. I have been able to get within about 30 feet or so of most corners, which is not bad for a property line about 3000 feet long, and you can't see for more than 50 feet in some places. And after doing the averaging thing a lot more accurate than the plat drawn in '33. BTW, I don't think surveyors use a transit much anymore. They use this electronic thing which shoots a laser to a prism and back, then times it to calculate the distance. The unit has a built in computer and is accurate to a few millimeters in about 30 miles. I am NOT surveyor, so please don't crucify me.
|
